Recently, in view of sharp rise of crude oil price since oil crisis, demands for versatility of sources of energy or the like, the drilling for oil and a natural gas and the pipeline construction in a very cold land such as the North Sea, Canada and Alaska have been actively promoted. Further, with respect to a pipeline, there has been observed a trend where a high-pressure operation is performed using a large-diameter pipe to enhance transport efficiency of a natural gas or oil. To withstand a high-pressure operation in a pipeline, it is necessary to forma transport pipe (line pipe) using a thick steel pipe so that a UOE steel pipe which uses a plate as a raw material is used.
Recently, however, along with strong demands for the further reduction of construction cost of a pipeline, demands for the reduction of a material cost of steel pipes are strong. Accordingly, as a transport pipe, in place of a UOE steel pipe which uses a plate as a raw material, a high strength welded steel pipe which is formed using a coil-shaped hot rolled steel sheet (hot rolled steel strip) which possesses high productivity and can be produced at a lower cost as a raw material has been used.
These high strength welded steel pipes are required to possess high strength and, at the same time, excellent low-temperature toughness from a viewpoint of preventing bust-up of a line pipe. To manufacture such a steel pipe which possesses both of high strength and high toughness, attempts have been made to impart higher strength to a steel sheet which is a raw material of a steel pipe by transformation strengthening which makes use of accelerated cooling after hot rolling, precipitation strengthening which makes use of precipitates of alloy elements such as Nb, V, Ti or the like, and attempts have been made to impart higher toughness to the steel sheet through the formation of microstructure by making use of controlled rolling or the like.
Further, a transport pipe (line pipe) which is used for transporting crude oil or a natural gas which contains hydrogen sulfide is required to be excellent in so-called sour gas resistances such as hydrogen introduced cracking resistance (HIC resistance) or stress corrosion cracking resistance in addition to properties such as high strength and high toughness.
To satisfy such a demand, patent document 1, for example, proposes a method of manufacturing a high strength line-pipe-use steel sheet which possesses excellent hydrogen induced cracking resistance. A technique disclosed in patent document 1 is directed to a method of manufacturing a steel sheet for a high-strength electric resistance welded steel pipe of APIX 70 grade or more. That is, patent document 1 describes a method of manufacturing a steel sheet for a high-strength line pipe having excellent hydrogen induced cracking resistance, wherein a slab is heated at a temperature of 1000° C. to 1200° C. and is subjected to hot rolling thus forming a steel sheet, the steel sheet is cooled down such that a surface temperature of the steel sheet becomes a temperature of 500° C. or below by accelerated cooling after hot rolling is finished, the accelerated cooling is stopped once and the steel sheet is reheated such that the surface temperature of the steel sheet becomes a temperature of 500° C. or above and, thereafter, the steel sheet is cooled down to a temperature of 600° C. or below by accelerated cooling at a cooling rate of 3 to 50° C./s. The technique described in patent document 1 adopts intermittent accelerated cooling so that the temperature distribution in the steel sheet becomes uniform in the sheet thickness direction and, at the same time, the hardened structure formed on a surface side is subjected to tempering so that the hydrogen induced cracking resistance of a high strength steel sheet can be enhanced while suppressing the increase of hardness of the steel sheet in the vicinity of a surface of the steel sheet.
Further, patent document 2 proposes a method of manufacturing a high strength steel plate which possesses excellent hydrogen induced cracking resistance. A technique disclosed in patent document 2 is directed to a method of manufacturing a steel sheet for a high-strength steel pipe of APIX 60 grade or more. That is, patent document 2 describes a method of manufacturing a high strength steel plate having excellent hydrogen induced cracking resistance, wherein a slab is heated at a temperature of 1000° C. to 1200° C., the slab is subjected to rolling at a reduction rate of 60% or more in an austenite temperature range of 950° C. or below, a steel plate formed by rolling is cooled from (Ar3-50° C.) or above until a surface temperature of the steel plate becomes 500° C. or below at an average cooling rate of 5 to 20° C./s at a center portion of the steel plate, and the steel plate is cooled to 600° C. or below at an average cooling rate of 5 to 50° C./s at the center portion of the steel plate. The technique described in patent document 2 adopts two-stage cooling which changes a cooling rate in the midst of cooling so that the steel plate can secure desired strength while suppressing hardness of the steel plate in the vicinity of a surface of the steel plate.